In many hydrometallurgical processes for recovering nickel and cobalt from their sulfidic or oxidic ores, intermediate aqueous process streams are generated which contain dissolved nickel and cobalt. In order to produce high purity cobalt from such solutions it is necessary to remove the nickel therefrom down to very low levels. For example, cobalt products are desired, the purity of which is such that the cobalt to nickel ratio is of the order of 1000 to 1. If such cobalt is to be produced from the purified solution, by hydrogen reduction for example, the solution itself must exhibit a purity of Co:Ni&gt;1000:1 if the desired product purity is to be achieved. It would be particularly advantageous to be able to use an ion exchange procedure for purifying the cobalt-containing solutions.
Many ion exchange resins available commercially are said to exhibit some selectivity between nickel and cobalt, i.e., nickel will load more readily than cobalt into such resins. The selectivity indicated by the published literature for such resins is of a smaller degree than that which is achievable between more dissimilar metal pairs, such as nickel and copper or nickel and zinc. Nevertheless, some of the resins which have been developed in recent years would seem capable of being applied to the task of cobalt solution purification. Selection of any resin is in practice based primarily on the Ni/Co selectivity factor quoted for such resin, or calculable from the quoted data. The selectivity factor, S.sub.Ni/Co, is defined as the ratio of distribution coefficients D.sub.Ni and D.sub.Co, each of which in turn is the ratio of the concentration of metal in the resin and in an aqueous solution in equilibrium therewith. Thus the selectivity factor can be expressed as: ##EQU1## wherein [ ].sub.r refers to the metal concentration in the resin, and [ ].sub.a refers to the metal concentration in the aqueous phase in equilibrium therewith.
In the case of some of recently developed resins, the Ni/Co selectivity can be estimated from selectivity published for each of these two metals with respect to a common reference, typically calcium. When the claimed selectivity is examined in this way, the most promising of commercial resins would seem to be a chelating resin having aminocarboxylic acid functional groups, available from Rohm and Haas under the name: Amberlite* IRC718. The selectivity factor for this resin is indicated by the manufacturer's trade literature as being 54.4 (evaluated from selectivity factors of 3100 and 57 quoted for nickel and cobalt, respectively, with respect to calcium). Indeed the use of such a resin for selective recovery of nickel and cobalt from aqueous solutions is the subject of U.S. Pat. No. 4,123,260 issued on Oct. 31, 1978 to Sefton and Kofluk. FNT *Trademark
It has now been found, however, that resins of the aminocarboxylate type are not capable of removing nickel effectively from concentrated cobaltous solution. Specifically, it was determined that the selectivity exhibited by a resin such as Amberlite IRC718 is lower (by as much as an order of magnitude) in the presence of concentrated aqueous cobaltous solutions, than the value indicated by the published literature, which value is probably related to resin performance in the presence of dilute solutions such as effluent streams from which small amounts of Ni or Co are to be removed.